This invention generally relates to the determination of blood flow through a body segment by detecting changes in the impedance of the segment which result from the blood flow. The invention has the particular utility in the determination of cardiac output.
Prior methods of determining blood flow from body impedance changes usually include subjecting a body segment to a high frequency electrical current and measuring the resultant voltage variations which represent impedance variations caused by the blood flow through the body segment. The impedance method of blood flow detection, often called impedance plethysmography, is based on the fact that blood has a much higher conductivity than the muscle, bone, and other tissue in the body segment. When the body segment is subjected to a high frequency electrical current, the changes in impedance are inversely proportional to the amount of blood therein. Prior systems for detecting impedance changes caused by blood flow or blood volume variations are generally described in the following list of references which is considered exemplary not exhaustive.
______________________________________ PATENT NO. PATENTEE ______________________________________ Re30,101 Kubicek et al. U.S. Pat. No. 3,882,851 Sigworth U.S. Pat. No. 3,871,359 Pacela U.S. Pat. No. 3,994,284 Voelker U.S. Pat. No. 3,996,925 Djordjevich U.S. Pat. No. 4,450,527 Sramek ______________________________________
For further information, reference is also made to the article by Kubicek et al. "The Minnesota Impedance Cardiography - Theory and Applications," Biochem Eng. 9:410, 1974 which describes some of the basic work done on impedance detection to determine cardiac output.
As is evident from the above references, high frequency electrical current is usually applied to the body segment by means of spaced apart transmitting electrodes which are positioned on the surface of the body segment in question. The impedance of the body segment is determined by spaced apart sensing electrodes which are disposed on the surface of the body segment between the two current transmitting electrodes. Both spot and wrap-around electrodes have been described in the prior art. The sensing electrodes usually measure voltage which is directly proportional to the impedance.
As indicated in U.S. Pat. No. 4,450,527 (Sramek), one of the major problems in determining cardiac output by detecting changes in thoracic impedance is to minimize the effects of pulmonary expansion and contraction on thoracic impedance because the pulmonary effects greatly exceed the effects of cardiac output on thoracic impedance. The prior art methods of electronically and/or mathetmatically manipulating the sensed impedance signal to reduce the pulmonary effects has improved cardiac output determinations. However, this method has not met with widespread acceptance due in part to the fact that the sensing system may not have the sensitivity to accurately determine cardiac output. Moreover, the methods described are not very suitable for measuring cardiac output during thoracic surgery.
Thus, a substantial need remains for accurately determining cardiac output, particularly during thoracic surgery. The present invention responds to this need by providing a sensing system having greater sensitivity for detecting impedance and one which may be used during thoracic surgery.